Circular saws with a plunge-cut action are typically known as plunge saws. Such saws comprise a motor and a gearbox disposed in a housing which includes a handle and a manually operated switch for activating the motor. The housing is pivotally attached to a base plate which includes a blade guard for accommodating a saw blade which is attached to the gearbox's output spindle. Springs are arranged to urge the housing into a position where the saw blade is wholly accommodated within the blade guard. During use, a user places the base plate on a workpiece, depresses the switch to activate the motor thereby initiating the saw blade's rotation, and then plunges the housing towards the workpiece and base plate such that the rotating saw blade passes through an aperture in the base plate and into the workpiece. From this position, the saw can be moved in a forward direction along a cut-line thereby cutting a slot in the workpiece. DE19635527 describes such a saw.
Kickback can occur as the saw is plunged into the workpiece. The kickback phenomenon is not wholly understood, but it often occurs in the early stages of plunging the blade into the workpiece. As the saw blade first engages the workpiece, kick-back can cause the saw to jump out of the workpiece and back towards the operator with sufficient force to hit the operator. Kick-back might also occur if the saw is twisted out of alignment with the cut during cutting so that the blade catches the edge of the slot cut by the blade. As kick-back occurs, it is thought that the blade “grabs” the workpiece in the slot being cut. In the worse case scenario, the blade can stop rotating with respect to the workpiece and, as the motor continues to drive the blade, the saw is thrown off the workpiece and towards the user at high speed. The user is typically unable to react when kick-back occurs and has insufficient time to disengage power to the motor.
Thus, kickback is potentially dangerous and could cause severe injury. Furthermore, if a guide is being used to direct the cutting action of the saw as kickback occurs, the saw can jump out of the workpiece and across the guide while the blade is exposed from the blade guard and still rotating thereby damaging the guide and/or saw blade.
Guides are known and can be used to direct various power tools, such as circular saws, plunge saws, routers or jigsaws. The guide can be placed on a workpiece and the tool is then placed on the guide. Clamping devices are often used to secure the guide to the workpiece. Typically, a guide comprises a length of extruded aluminium having a rail which extends along the length of the guide parallel to an edge of the guide along which the power tool operates. In the case of a circular saw, the rail engages with a channel formed in the base plate of the saw. The saw can be placed on the guide such that the rail engages the channel. The operator can run the saw along the guide, directed by the rail, while the saw blade cuts the workpiece. A strip of material having a relatively high coefficient of fiction is disposed on the underside of the guide which engages the workpiece to maintain the guide in position during operation of the power tool. This is particularly useful if clamping devices are not available.
EP1410818A describes a guide comprising a stop part with an overlapping portion. A plunge saw base plate has a protruding tang which fits between the guide's rail and an overlapping portion of the stop part. The stop part can be disposed in a channel running along the length of the guide fixed in position with a thumbscrew arrangement. Before plunging the saw into a workpiece, the user places the guide on the workpiece and arranges the stop part in the desired position. The base plate of the saw is then positioned on the guide such that the tang abuts the stop part. Thus, if kickback occurs, the base plate is held on the guide and prevented from jumping backwards towards the operator by the stop part. The stop part and tang combine to form a manually adjusted anti-kickback device.
Another anti-kickback device is disclosed in EP1728604A for arresting movement of a rail accommodated within a channel. The device comprises a slide surface located in one of the rail or channel and is inclined in relation to an opposing side wall of the other of the rail or channel. The device comprises a slide member arranged to slide along the slide surface between an extended position and a retracted position further from the side wall than the extended position. The device further comprises a resilient member which biases the slide member along the slide surface towards the extended position and into abutment with the side wall. Movement of the channel in relation to the rail in a longitudinal forward direction causes friction between the slide member and the side wall to push the slide member along the slide surface and towards the retracted position. Movement of the channel in relation to the rail in a longitudinal backward direction opposite to the forward direction causes friction between the slide member and the side wall to push the slide member along the slide surface towards the extended position thereby taking up clearance between the rail and the channel to the extent that the rail is wedged stationary against the channel.
As mentioned above, the purpose of such a device is to prevent unexpected and sudden movement of the saw towards the user. Thus, should kick-back occur, the saw is prevented from moving back in a longitudinal backward direction along the rail and towards the operator. Such a device can be integral with a component comprising the rail, like, for example a guide. Alternatively, such a device can be integral with a component comprising the channel, like, for example the base plate of a saw, or the underside of a saw.
The user need only engage the rail and the channel for the anti-kickback device to be ready for action. Thus, no manual adjustment or activation is required and the user will ordinarily assume that the device functions properly. This makes the reliability of the device all the more important because, normally, the user has no need to check it before use.
The present invention provides an anti-kickback device of the type disclosed in EP1728604A whereby the resilient member is at least partially received within a hollow elongate sleeve. A resilient member, by its very nature, becomes less stable the more it is compressed. A good example would be a helical spring which is liable to bow laterally outwardly when compressed along its longitudinal axis. The same could apply to a cylindrical rubber bushing. The risk of bowing outwardly is reduced, or eliminated, by the hollow elongate sleeve which braces the resilient member in its lateral direction while allowing the resilient member freedom to be compressed, or relaxed, along its longitudinal axis. This helps to prevent the resilient member from detaching itself from the device. The sleeve also helps to house the resilient member where it is safe from interference or damage.
Preferably, the sleeve is a hollow cylinder and the resilient member is a helical spring. A helical spring is a freely available item which can be manufactured small enough to be housed in a compact space. The hollow cylinder braces the helical spring when it is compressed inside the sleeve which helps prevent the spring from bowing laterally outwardly.
Preferably, the sleeve is guided along a straight path by a guidance mechanism. The guidance mechanism is part of the device and prevents the sleeve from moving laterally outwardly from the device. This further enhances the ability of the sleeve to prevent the spring from bowing laterally outwardly. That is because the sleeve can only travel along the straight path and so the helical spring, which is braced by the sleeve, cannot stray from the straight path and leave the device.
The guidance mechanism may be any system capable of guiding the sleeve along a straight path. Preferably, the guidance mechanism comprises a ridge on the sleeve which is slideable within with an elongate groove in the one of the rail or channel. Alternatively, the guidance mechanism comprises a ridge in the one of the rail or channel which is slideable within with an elongate groove on the sleeve. Either of these two variants is a simple and inexpensive guidance mechanism.
Preferably, there is a ridge on opposite sides of the sleeve, wherein each ridge has a corresponding groove. Having ridges on opposing sides of the sleeve improves stability as the sleeve moves along the straight path because it is guided on two diametrically opposing sides and is less likely to become stuck.
Preferably, the slide member is a cylindrical roller with a central axis arranged parallel to the slide surface. The lines of contact between the cylindrical roller and the slide surface, on the one hand, and between the cylindrical roller and the rail or the channel, on the other hand, make the roller less prone to jamming than a face-to-face contact, as would be the case of the slide member were, for example, cube-shaped.
Preferably, the device further comprises a manually operable button coupled to the slide member to enable user-operated movement of the slide member towards the retracted position. This may be necessary in the rare event that the slide member is wedged stuck between the slide surface and the side wall causing the rail to be immobilised in relation to the channel. This may also be necessary in the more common event where the user wishes to suspend operation of the device and allow free movement of the channel in relation to the rail in a longitudinal backward direction. For example, when the user wishes to slide the plunge saw in the longitudinal direction back to its start position and without lifting it from the guide. With the slide member forced into the retracted position there can be no friction between the slide member and the side wall.
Preferably, the device further comprises a lock mechanism for holding the slide member in the retracted position. This is the case when a user wishes to temporarily suspend operation of the device and allow free movement of the channel in relation to the rail in both longitudinal directions. The lock does this while freeing both the user's hands to slide the plunge saw back to its start position and without the need to lift it from the guide.
Preferably, the lock mechanism comprises a hook on the button for engagement with a dowel on the one of the rail or channel. This is a simple and inexpensive lock mechanism. It can rely on the bias of the resilient member to maintain the hook engaged with the dowel if the dowel is arranged on the opposite side of the button to the direction of bias of the resilient member.
Preferably, the button is coupled to the slide member by a pin passing through a linear slot in the one of the rail or channel, wherein the slot is parallel to the slide surface. Thus, the slot and pin arrangement do not interfere with the slide member's contact with the slide surface.
The present invention also provides a hand-operated tool guidable by a rail, the tool comprising a main housing, a base plate coupled to the main housing, and an elongate channel in one side of the base plate, wherein the channel is arranged to accommodate a rail and the base plate comprises the anti-kickback device.
The present invention further provides a guide for guiding a hand-operated tool, the guide comprising: a support surface, and a rail protruding from one side of the support surface, wherein rail is arranged to be accommodated within an elongate channel in one side of a tool and the guide comprises the anti-kickback device.